CMOS Buffer

Library

Description

The block output logic level is HIGH if
the logic level of the gate input is 1.

The block output logic level is LOW otherwise.

The block determines the logic levels of the gate inputs as
follows:

If the gate voltage is greater than the threshold
voltage, the block interprets the input as logic 1.

Otherwise, the block interprets the input as logic
0.

The threshold voltage is the voltage
value at midpoint between the High level input voltage parameter
value and the Low level input voltage parameter
value.

Note

To improve simulation speed, the block does not model all the
internal individual MOSFET devices that make up the gate. See Basic Assumptions and Limitations for
details.

The block models the gate as follows:

The gate inputs have infinite resistance and finite
or zero capacitance.

The gate output offers a selection of two models: Linear and Quadratic.
For more information, see Selecting the Output Model for Logic Blocks. Use the Output
current-voltage relationship parameter to specify the output
model.

You can specify propagation delay for both output
models. For Linear output, the block sets
the value of the gate output capacitor such that the resistor-capacitor
time constant equals the Propagation delay parameter
value. For Quadratic output, the gate input
demand is lagged to approximate the Propagation delay parameter
value.

The block output voltage depends on the output model selected:

For Linear model, output
high is the High level output voltage parameter
value, and output low is the Low level output voltage parameter
value.

For Quadratic model, the
output voltage for High and Low states is a function of the output
current, as explained in Quadratic Model Output and Parameters. For zero load
current, output high is Vcc (the Supply voltage parameter
value), and output low is zero volts.

Basic Assumptions and Limitations

The block does not model the internal individual MOSFET devices
that make up the gate (except for the final MOSFET pair if you select
the Quadratic option for the Output
current-voltage relationship parameter). This limitation
has the following implications:

The block does not accurately model the gate's response
to input noise and inputs that are around the logic threshold voltage.

The block does not accurately model dynamic response.

Circuits that involve a feedback path around a set of logic
gates may require a nonzero propagation delay to be set on one or
more gates.

Parameters

Inputs Tab

Voltage value below which the block interprets the input voltage
as logic LOW. The default value is 2 V.

High level input voltage

Voltage value above which the block interprets the input voltage
as logic HIGH. The default value is 3 V.

Average input capacitance

Fixed capacitance that approximates the input capacitance for
a MOSFET gate. The MOSFET capacitance depends on the applied voltage.
When you drive this block with another gate, the Average
input capacitance produces a rise time similar to that
of the MOSFET. You can usually find this capacitance value on a manufacturer
datasheet. The default value is 5 pF. Setting this
value to zero may result in faster simulation times.

Outputs Tab

Output current-voltage relationship

Select the output model, Linear or Quadratic.
The default value is Linear.

Low level output voltage

Voltage value at the output when the output logic level is LOW.
The default value is 0 V. This parameter is available
when you select the Linear option for the Output
current-voltage relationship parameter.

High level output voltage

Voltage value at the output when the output logic level is HIGH.
The default value is 5 V. This parameter is available
when you select the Linear option for the Output
current-voltage relationship parameter.

Output resistance

Value of the series output resistor that is used to model the
drop in output voltage resulting from the output current. The default
value is 25 Ω. You can derive this value
from a datasheet by dividing the high-level output voltage by the
maximum low-level output current. This parameter is available when
you select the Linear option for the Output
current-voltage relationship parameter.

Supply voltage

Supply voltage value applied to the gate in your circuit. The
default value is 5 V. This parameter is available
when you select the Quadratic option for
the Output current-voltage relationship parameter.

Measurement voltage

The gate supply voltage for which mask data output resistances
and currents are defined. The default value is 5 V.
This parameter is available when you select the Quadratic option
for the Output current-voltage relationship parameter.

Logic HIGH output resistance at zero
current and at I_OH

A row vector [ R_OH1R_OH2 ]
of two resistance values. The first value R_OH1 is
the gradient of the output voltage-current relationship when the gate
is logic HIGH and there is no output current. The second value R_OH2 is
the gradient of the output voltage-current relationship when the gate
is logic HIGH and the output current is I_OH. The
default value is [ 25 250 ] Ω. This parameter
is available when you select the Quadratic option
for the Output current-voltage relationship parameter.

Logic HIGH output current I_OH when
shorted to ground

The resulting current when the gate is in the logic HIGH state,
but the load forces the output voltage to zero. The default value
is 63 mA. This parameter is available when you
select the Quadratic option for the Output
current-voltage relationship parameter.

Logic LOW output resistance at zero
current and at I_OL

A row vector [ R_OL1R_OL2 ]
of two resistance values. The first value R_OL1 is
the gradient of the output voltage-current relationship when the gate
is logic LOW and there is no output current. The second value R_OL2 is
the gradient of the output voltage-current relationship when the gate
is logic LOW and the output current is I_OL. The
default value is [ 30 800 ] Ω. This parameter
is available when you select the Quadratic option
for the Output current-voltage relationship parameter.

Logic LOW output current I_OL when shorted
to Vcc

The resulting current when the gate is in the logic LOW state,
but the load forces the output voltage to the supply voltage Vcc.
The default value is -45 mA. This parameter is
available when you select the Quadratic option
for the Output current-voltage relationship parameter.

Propagation delay

Time it takes for the output to swing from LOW to HIGH or HIGH to LOW after
the input logic levels change. The default value is 25 ns.

Protection diode on resistance

The gradient of the voltage-current relationship for the protection
diodes when forward biased. The default value is 5 Ω.
This parameter is available when you select the Quadratic option
for the Output current-voltage relationship parameter.

Protection diode forward voltage

The voltage above which the protection diode is turned on. The
default value is 0.6 V. This parameter is available
when you select the Quadratic option for
the Output current-voltage relationship parameter.

Initial Conditions Tab

Output initial state

Specify whether the initial output state of the block is High or Low.
This parameter is used for both linear and quadratic output states,
provided that the Propagation delay parameter
is greater than zero and the Solver Configuration block
does not have the Start simulation from steady state option
selected. The default value is Low.

Ports

The block has the following ports:

A

Electrical input port

J

Electrical output port

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